A conventional system of this kind is disclosed in Japanese Patent Laid-Open No. 190240/1982 and shown in in FIG. 7, where the system has an input shaft 1 and an output shaft 2. The output shaft 2 has a hole in which one end of the input shaft 1 is fitted such that the shafts 1 and 2 can rotate relative to each other A cylindrical movable member 3 is mounted on the outer periphery of the output shaft 2 so as to be slidable The movable member 3 is provided with slots 3a and 3b extending vertically. Pins 4 and 5 rigidly fixed to the output shaft 2 can fit in the slots 3a and 3b, respectively. Thus, the movable member 3 is able to vertically move relative to the output shaft 2 but unable to rotate relative to the output shaft 2 A pin 6 is firmly fixed to the input shaft 1 and extends through a hole 2a formed in the output shaft 2. the pin 6 is fitted in a slot 3c which is formed in the movable member 3 in a helical direction of the member 3 so that the pin 6 can slide in the slot 3c. When the input shaft 1 makes an angular movement relative to the output shaft 3, the movable member 3 is shifted vertically. A fixed sensor 7 is disposed on the outside of the movable member 3 to detect the vertical position of the member 3. The input shaft 1 and the output shaft 2 are allowed to move vertically, i.e., axially, relative to each other by the pin 6 and the slot 3c.
In the conventional structure constructed as described above, the input shaft 1 and the output shaft 2 vertically move relative to each other because of the clearance between the outer surface of the pin 6 and the lateral sides of the slot 3c, i.e., they rattle. This makes the vertical position of the movable member 3 unstable, thus adversely affecting the output from the sensor 7. If the clearance between the pin 6 and the slot 3c is narrowed to avoid these difficulties, another problem will take place. Specifically, when the input shaft 1 and the output shaft 3 twist relative to each other because of the clearance existing between the mating portions of the shafts 1 and 2, the outer surface of the pin 6 may twist in the slot 3c. This prevents the pin 6 and the movable member 3 from sliding smoothly, thus adversely affecting the output from the sensor 7.